This invention is directed to fiber optic couplers and in particular to couplers for multimode optical fibers and a method of producing these couplers.
Multimode optical fiber transmits optical energy in more than one optical mode. Various methods have been used to produce multimode optical couplers. The simplest method involves cleaving two optical fibers normal to the longitudinal axis and aligning them end to end with a third fiber to form a "Y" coupler. Disadvantages of the Y type coupler include the fact that the fibers must be carefully cleaved at the proper angle; alignment of the fibers is critical and often difficult to achieve and these couplers generally exhibit large excess losses. Another method of forming a multimode coupler involves etching away the cladding layers of two multimode fibers. The etched fibers are then twisted together to achieve evanescent wave coupling. Long coupling regions are required to obtain significant coupling of optical energy from one fiber to the other.
U.S. Pat. No. 4,291,940 to Kawasaki et al. describes a fused biconical taper coupler. This coupler is fabricated by forming biconical taper sections in two portions of two multimode fibers, twisting the fibers together and then heating a region of the tapered section to fuse them together. Problems with this method include the fact that twisting of the fibers often causes increased excess loss. It is also more difficult to achieve consistent tapers from one coupler to another. And in this particular coupler typical losses range from one to two decibels.
Twisting the fibers together has two major drawbacks: (1) the twist tends to induce microbend loss in the fibers and increases the excess loss of the coupler; (2) the inventors' experience has shown that the twist also tends to make the taper process more difficult to control. In the twisted region the fibers are in contact over a very short length and this may cause the tapers to be too steep. If the tapers are too steep or sharp, the loss resulting from the tapering process can become severe. The inventors have also observed that the twist can move along the fiber lengths if the tension in the fibers is not balanced. If the twisted region moves away from the center of the heat source during fusion or is simply located off-center, the resulting taper profile changes. Hence, unless the twist is carefully done, it can be a troublesome source of inconsistent and nonreproducible tapers.
Therefore it is an object of the present invention to provide a multimode coupler having losses of the order of less than one decibel.
It is another object of the invention to provide a method of making a multimode coupler which is simple and reproducible and can produce couplers having stable performance characteristics.
It is yet another object of the present invention to provide a reproducible method of making low loss fused multimode fiber couplers.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.